Charge control agents for liquid developers

ABSTRACT

Liquid developer for developing an electrostatic latent image present on an electrostatographic imaging surface comprising an insulating liquid having dispersed therein positively charged marking particles and also containing at least one charge control agent selected from the group consisting of vinyltriethoxysilane, gamma -glycidoxypropyltrimethoxysilane, and Beta -(3,4epoxycyclohexyl)ethyltrimethoxysilane.

Unite States Patent [1 1 Honjo et al.

[ CHARGE CONTROL AGENTS FOR LIQUID DEVELOPERS [75] Inventors: SatoruHonjo, Tokyo; Hajime Miyatuka, Asaka; Sadao Osawa, Kawaguchi, all ofJapan [73] Assignee: Rank Xerox, Ltd., London, England [22] Filed: Oct.13, 1972 [21] Appl. No.: 298,721

Related U.S'. Application Data [62] Division of Ser. No. 121,537, March5, 1971, Pat.

[30] Foreign Application Priority Data Mar. 12, 1970 Japan 45-20984 [52]US. Cl. 117/37 LE, 252/621 [51] Int. Cl. G03g 9/04, (103g 13/10 [58]Field of Search 117/37 LE; 96/1 LY;

[56] References Cited UNITED STATES PATENTS 2,989,412 6/1961 Koehler etal. 106/30 [4 Oct. 15, 1974 2/1962 Te Grotenheis 106/30 3,053,688 9/1962Greig 252/621 3,098,833 7/1963 Solomon 252/621 3,417,019 12/1968 Beyer252/621 3,427,247 2/1969 Peck 252/621 3,522,181 7/1970 Garrett ct al.252/621 3,528,097 9/1970 Otsuka ct a1 252/621 3,729,419 4/1973 Honjo ctal. 117/37 LE Primary ExaminerMichael Sofocleous 5 7 ABSTRACT 6 Claims,N0 Drawings CHARGE CONTROL AGENTS FOR LIQUID DEVELOPERS BACKGROUND orTHE INVENTION This invention relates to liquid developers for use inelectrostatographic imaging systems, and more particularly, to improvedliquid developers with novel charge control agents.

The formation and development of images on the surface of photoconductormaterial by electrostatic means is well known. The basicelectrostatographic process as taught by C. F. Carlson in U.S. Pat. No.2,297,691 involves placing a uniform electrostatic charge on aphotoconductive insulating layer, exposing the layer to a light andshadow image to dissipate the charge on the areas of the layer exposedto the light and developing the resulting electrostatic latent image bydepositing on the image a finely divided electroscopic marking materialreferred to in the art as toner. The toner will normally be attracted tothoseareas of the layer which retain a charge thereby forming a tonerimage corresponding to the electrostatic latent image. The powder imagemay then be transferred to a support surface such as paper andpermanently affixed to the support by any suitable means such as heatfixing or solvent fixing. Alternatively, the powder image may be fixedto the photoconductive layer if elimination of the powder transfer stepis desired. In addition, instead of latent image formation by uniformcharging followed by imagewise exposure, the latent image may be formedby directly charging the layer in image configuration. Other methods areknown for applying electroscopic particles to the imaging surface.Included within this group are the cascade development techniquedisclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the powder clouddevelopment technique disclosed byC. F. Carlson in U.S. Pat. No.2,221,776; and the magnetic brush process disclosed, for example,-inU.S. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved withliquid rather than dry developer materials. In conventional liquiddevelopment, more commonly referred to as electrophoretic development,an insulating liquid vehicle having finely divided solid materialdispersed therein contacts theimaging surface in both charged anduncharged areas. Under the influence of the electric field associatedwith a charged image pattern, the suspended particles migrate toward thecharged portions of the imaging surface separating outof the insulatingliquid. This electrophoretic migration of charged particles results inthe deposition of the charged particles on the imaging surface in imageconfiguration. Electrophoretic development of an electrostatic latentimage may, for example, be obtained by pouring the developer over theimage bearing surface,

by immersing the imaging surface'in a poolof the de-' veloper or bypresenting the liquid developer on a smooth surface roller and movingthe roller against the imaging surface. The liquid development techniquehas been shown to provide developed images of excellent quality and toprovide particular advantages over other development methods in offeringease in handling.

Typically, the electrostatographic liquid developers are generallydispersions comprising a highly insulating liquid referred to as thecarrier liquid and dispersed therein submicron size marking particleswhich may comprise a pigment. In addition, fixing agents such asresinous materials to assist in fixing the toner particles to thesupport member and suspending or stabilizing agents may be added toinsure uniform suspension of the marking particles throughout theinsulating liquid.

To'formulate the developer, the marking particles are first dispersedwithin the continuous phase of any resinous material. Conventionalballmill, three roll mill or sand mill may be employed to provide ahomogeneous mixture of resinous material and pigment which is sometimesreferred to as apaste or concentrate which may be subsequently dilutedwith carrier liquid or dispersed in a large volume of a carrier liquidto form the liquid developer. Inaddition, if desired to improve thedispersion stability of the developer, other additives such as surfaceactive agents may be added.

Furthermore, if desired, one may use concentrated dispersionscommercially available such as printing ink,

tars or pitches in which the pigment particles are already dispersed ina resinous vehicle. Stable dilute dispersions of these can be preparedmerely by dissolving the paste in carrier liquids by means-of, forexample, ultrasonic devices.

While a satisfactory degree of dispersion and dispersion stability maybe readily achieved in general most liquid developers other necessarycharacteristics of the liquid developer are frequently difficult toachieve and to maintain for. an adequate period of time. Sincedevelopment is achieved by the migration of charged particles throughthe insulating liquid in response to the charge pattern on the imagingsurface charge of proper polarity and magnitude should be maintained onthe charged particles. In practice, however, considerable difficulty,has been experienced in maintaining this polarity and magnitude ofcharge on the individual marking particles. The reasons for thealteration in polarity and magnitude of charge on the charged particlesare many and include internal and external'influences on the liquiddeveloper. In order to control the polarity and the magnitude of chargeon the individual marking particles, the practice of adding chargecontrol agents to the liquid developer has been employed. Very oftenthese control agents may also function for stabilizing or fixingproperties. The choice of materials, particularly resinous materials,suitable for charge control agents is rather limited. conventionally,alkyd resins and linseed oil have served the dual purpose in liquiddevelopers of. functioning not only as the vehicle for the pigments, butalso as the desired charge control agent to impart a positive charge tothe dispersed particles. However, the suitability of these liquiddevelopers is .limited by the specific materials and compositions whichare capable-of producing the necessary charge control and theflexibility in developer formulation is thereby limited. Theselimitations are even more strict for printing inks, tars or pitches.

In addition, other additives frequently are employed to improve theelectrophoretic performance of liquid developer. ,Typically, theseadditives are soluble in the carrier liquid and exhibit a marked chargedcontrolling capability without reducing the volume resistivity of theliquid developer to a value lower than the critical threshold. However.only a limited number of compounds such as cobalt naphthenate, andcopper oleate are capable of meeting these requirements. Thus, there isa continuing need for additional and more: effective charge controllingagents for electrostatographic liquid developers.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide liquid developers which overcome the above noted deficiencies.

It is another object of this invention to provide novel charge controlagents for liquid developers.

It is another object of this invention to provide liquid developers ofhigh electrical resistivity including'positively charged tonerparticles.

It is another object of this invention to provide a liquid developercapable of maintaining the desired polarity and magnitude of charge onthe individual marking particles.

It is another object of this invention to provide liquid developers ofincreased flexibility in compositions which are easy to formulate.

It is another object of this invention to provide an imaging methodcapable of producing clear high density prints.

It is another object of this invention to'provide a liquid developmentmethod capable of consistent print quality. a

The above objects and others are accomplished, generally speaking, byemploying as a charge controlling agent in electrostatographic liquiddevelopers selected organosilicon compounds. More specifically, liquiddevelopers of the present invention are characterized in that'theycontain at least one compound selected from the group consisting ofvinyltriethoxysilane, 'y-glycidoxypropyltrimethoxysilane, and ,B-(3,4-epoxycyclohexyl)ethyltrimethoxysilane in amounts of from about 0.5 toabout 2.0 percent by volume of the developerf Since generally speaking,organosilicon compounds are sufficiently polar to dissolve in alcohol,acetone or waterthereby markedly decreasing the volume resistivity ofthe developer it is quite surprising that the three recitedorganosilicon materials are capable of functioning as superior chargecontrolling agents for electrostatographic liquid developers. Thiscapability is believed to be due to the fact that the materialsaccording to this invention are readily soluble in many nonpolarsolvents such as cyclohexane, kerosene, toluol, xylol and isoparaffinichydrocarbons.

A particularly important feature of the present invention lies in thefact that it provides liquid developer capable of excellent performancewithout requiring speeialized complicated manufacturing processes sincethe charge control function of the charge control agents of thisinvention may be achieved-merely by adding the recited compounds to thedispersion of marking particles in the insulating liquid. Furthermore,since the developer of the present invention includes only positivelycharged toner, an image of improved optical density may be obtained. Inaddition, since the charge control agents of the present invention areso effective in controlling the electrophoretic performance of theliquid developer a wider selection of other additives and particularlyresinous materials is possible. The resinous materials which in the pastwere unacceptable since they would not function alone adequately intheir charge controlling properties cannow be used in conjunction withthe charge control agents of this invention to provide superior liquiddevelopers. Furthermore, the charge control agents of the presentinvention may be used to improve the elcctrophoretic performances ofdevelopers made from commercially available printing inks, tars, orpitches. Since the charge control agents of the present invention may bereadily mixed with the liquid developer the range of raw materialscapable of use in liquid developers may in some instances be greatlyenlarged.

The organosilicon compounds may be present in the liquid developer inany suitable amount. if too small an amount is incorporated in theliquid developer, the charge controlling effect is inadequate. On theother hand, if too large an amount of charge control agent'is added tothe liquid developer, the volume resistivity of the developer may belowered to a degree such that on contact with the developer, anelectrostatic charge pattern may be discharged. In addition, in somecases, coagulation of marking particles may occur. Typically. the chargecontrol agent is present in an amount of from about 0.5 to about 2.0percent by volume of the developer. The charge control agents of thepresent invention may be added to any suitable electrostatographicliquid developers. Typically, the liquids employed have relatively highinsulating values generally having a volume resistivity greater thanabout 10" ohm-cm so as not to effect the electrostatic charge pattern onthe insulating layer and low dielectric constant of less than about 3.5.Typical specific vehicles include hydrocarbons such as benzene, xylene,hexane, naphtha, kerosene, cyclohexane, Decalin, isoparaffinichydrocarbons and halogenated hydrocarbons such as carbon tetrachloride,trichloroethylene, and chloroform. Typical electroscopic markingparticles include among others, charcoal, carbon black, magnesium oxide,lithopone, cadmium yellow, chrome yellow, cobalt blue, cadmium red,burnt siena, Hausa yellow, rose bengal and phthalocyanine which arepresent in an amount of from about 2 to about 20 grams per liter. Theelectroscopic marking particles are conventionally dispersed andsuspended in the liquid by stirring or agitation and where a highlyuniform and stable suspension is desired, this suspension may be passedthrough a colloid mill. As discussed above, if desired, suspending ordispersing agent may be added for their well known functions.

The liquid developer according to the present invention may be employedto develop an electrostatic charge pattern present on any suitableimaging surface. Basically, any material capable of holding the chargepattern may be employed. Typical materials include dielectric layers andphotoconduetors. A particularly preferred material for use in automaticcopiers is a photosensitive paper comprising photoconductive pigmentparticles in an insulating binder layer. Typically, this paper compriseszinc oxide photoconductive particles present in an insulating binderlayer which is overcoated on the paper substrate. The particular imagingmember and particular development technique may be readily determined byone skilled in the art. The photosensitive paper described above, forexample, may be substituted with photoconductive materials made fromcadmium sulfide, zinc sulfide, zinc selenide, cadmium selenide, titaniumdioxide, phthalocyanine and polyvinylcarbazole.

DESCRIPTION OF PREFERRED EMBODIMENTS The following preferred examplesfurther define and describe the preferred materials-and methodsof thepresent invention. Examples II, III and V are presented for comparativepurposes. Unless otherwise indicated, all parts and percentages are byweight.

EXAMPLE I An electrostatic latent image present on anelectrophotographic member which has been uniformly charged and exposedto a light and shadow pattern in conventional manner is developed bycontacting the surface with a liquid developer formed in the followingmanner: To 1,000 ml of kerosene, 5 grams of black offset printing inkJet King G process H Black (commercially available from Toyo InkManufacturing Company) is added and dispersed therein by means of anultrasonic dispersing device. To this dispersion 1.0 percent by volumeof'developer of vinyltriethoxysilane (available from Shin-etsu ChemicalIndustry under the tradename KBE 1003+) is added to provide a liquiddeveloper wherein the carbon black is positively charged. When appliedto the surface of the electrophotographic plate, a black tar imagehaving high optical density is obtained. Upon repeated development, nochange in print quality is observed.

EXAMPLE II The procedure of Example I is repeated except that thevinyltriethoxysilane is omitted from the liquid developer. While thetoner particles remain positively charged in this liquid developer, theelectrophoretic performance of the developer fluctuates from portion toportion of the original printing ink.

EXAMPLE III An electrostatic latent image of negative polarity is formedon an electrophotographic plate by charging the plate negatively andexposing it to a light and shadow pattern in conventional manner. Theelectrostatic latent image is developed with a liquid developer made inthe following manner: Two grams of channel black and 50 grams of analkyd resin purchased from Japan Reichold Chemical Industry under thetradename Beckosol EL 8002 are blended in a 500 ml ball mill jar forabout two days to prepare a concentrated paste which is subsequentlydispersed in about 2,000 ml of an isoparaffinie solvent (Isopar H,available from Humble Oil and Refining Company). The majority of thetoner particles in this liquid developer acquire a positive charge.However, particles bearing a negative charge are also present and whenthe developer is applied to the electrophotographic plate a low densitytoner image with hallow and streaks around high contrast image areas isobtained.

EXAMPLE IV The procedure of Example III is repeated except that about1.0 percent by volume of the developer of beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (available as KBM 303 fromShin-etsu Chemical Industries) is added to the liquid developerdescribed in Example III. Development of the electrostatic latent imageon the electrophotographic plate provides print of reduced hallow andstreaks as well as increased image density.

EXAMPLE V An electrostatic latent image present on anelectrophotographic plate which has been conventionally uniformlynegatively charged and exposed to a light and shadow pattern isdeveloped with a liquid developer prepared in the following manner: Ahomogeneous paste is prepared by blending about 200 grams of R4 varnishavailable from Toyo Ink Manufacturing Company and about 10 grams ofchannel black in a three roll mill. Fifty grams of this paste aredispersed in 2,000 mil of kerosene in an ultrasonic dispersing device.When used to develop the electrostatic latent image present on theelectrophotographic member, the electrophoretic performance of thisliquid developer is observed to change depending on the manufacturingconditions of the developer. In general, a very unclear low densityimage is produced.

EXAMPLE VI The procedure of Example I is repeated except that 1.5percent by volume of the developer of 'y-glycidoxypropyltrimethoxysilane(KBM 403" available from Shin-etsu Chemical Industries) is added to theliquid developer. When employed as a liquid developer in the developmentof electrostatic latent images present on electrophotographic layers, astable positively charged electrophoretic developer is observed toprovide consistent print quality with a marked reduction in performancefluctuation.

Although specific materials and operational techniques are set forth inthe above exemplary embodiments using the charge control agents andtechniques of this invention, they are merely intended as illustrationsof the present invention. There are other materials and techniques thanthose listed above which may be substituted for those in the exampleswith similar results. Other modifications of the present invention willoccur to those skilled in the art upon a reading of the presentdisclosure which modifications are intended to be included within thescope of this invention.

What is claimed is:

l. A method of developing an electrostatic latent image of negativepolarity present on an imaging surface comprising contacting saidimaging member with an electrostatographic liquid developer comprisingan insulating carrier liquid and uniformly dispersed therein finelydivided positively charged electroscopic marking particles, said liquiddeveloper further comprising an amount of a charge control agent whichis soluble in said carrier liquid and present in an amount of from about0.5 to about 2.0 percent by volume of the developer, said agent beingselected from the group consisting of vinyltriethoxysilane,'y-glycidoxy-propyltrimethoxysilane, andB-(3,4-epoxycyclohexyl)ethylitrimethoxysilane and mixtures thereofsufficient to pro vide a stable positive charge on the electroscopicmarking particles.

2.'A method of developing an electrostatic latent image according toclaim 1 wherein said liquid developer further comprises a resinousfixing agent. I

3. A method of developing an electrostatic latent image according toclaim 1 wherein said charge control agent is vinyltricthoxysilane.

4. A method of developing an electrostatic latent image according toclaim] wherein said charge control agent is'y-glycidoxypropyltrimethoxysilane.

8 image according to claim 1 wherein said electroscopic markingparticles are present in the liquid developer in an amount of from about2 to about 20 grams per liter.

1. A METHOD OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE OF NEGATIVEPOLARITY PRESENT ON AN IMAGING SURFACE COMPRISING CONTACTING SAIDIMAGING MEMBER WITH AN ELECTROSTATOGRAPHIC LIQUID DEVELOPER COMPRISINGAN INSULATING CARRIER LIQUID AND UNIFORMLY DISPERSED THEREIN FINELYDIVIDED POSITIVELY CHARGED ELECTROSCOPIC MARKING PARTICLES, SAID LIQUIDDEVELOPER FURTHER COMPRISING AN AMOUNT OF A CHARGE CONTROL AGENT WHICHIS SOSOLUBLE IN SAID CARRIER LIQUID AND PRESENT IN AN AMOUNT OF FROMABOUT 0.5 TO ABOUT 2.0 PERCENT BY VOLUME OF THE DEVELOPER, SAID AGENTBEING SELECTED FROM THE GROUP CONSISTING OF VINYLTRIETHOXYSILANE,Y-GLYCIDOXY-PROPYLTRIMETHOXYSILANE, ANDB(3,4-EPOXYCYCLOHEXYL)ETHYLITRIMETHOXYSILANE AND MIXTURES THEREOFSUFFICIENT TO PROVIDE A STABLE POSITIVE CHARGE ON THE ELECTROSCOPICMARKING PARTICLES.
 2. A method of developing an electrostatic latentimage according to claim 1 wherein said liquid developer furthercomprises a resinous fixing agent.
 3. A method of developing anelectrostatic latent image according to claim 1 wherein said chargecontrol agent is vinyltriethoxysilane.
 4. A method of developing anelectrostatic latent image according to claim 1 wherein said chargecontrol agent is gamma -glycidoxypropyltrimethoxysilane.
 5. A method ofdeveloping an electrostatic latent image according to claim 1 whereinsaid charge control agent is Beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.
 6. A method of developingan electrostatic latent image according to claim 1 wherein saidelectroscopic marking particles are present in the liquid developer inan amount of from about 2 to about 20 grams per liter.